Sir: Your editorial ("A quartet of unions that do the teachers a great disservice", 31 March) is wide of the mark. All the teachers unions are right to be objecting to the Green Paper.

There is no known way of measuring an individual teacher's impact on pupil performance. What is known is that there is a variety of factors which influence child development, not least the home environment. (And by recent reports, with over 25 per cent of children living in poverty, this is highly relevant, but ignored, as with the league tables.)

Governors and heads of schools are being asked to implement, from September, a pay-related-to-performance system which does not exist, and therefore, to do so without the skills, experience or time to make it work.

They will also be required to accept the judgements of external assessors and manage the consequences of the inconsistencies and unfairness in the application of differential pay scales and hours of work that will inevitably happen with decisions being made at 24,000 schools.